Overpressure-protected, differential pressure sensor

ABSTRACT

An overpressure-protected, differential pressure sensor (37) is formed by depositing diaphragm material (24) over a cavity (23) formed and filled with sacrificial material (22) into a front surface of a substrate. The sacrificial material (22) is then removed to create a free diaphragm. The floor of the cavity (23) defines a first pressure stop to limit the deflection of the diaphragm in response to pressure applied to the top of the diaphragm. A port (33) is created to allow pressure to be applied to the bottom side of the diaphragm (24). An optional second pressure stop, which limits the deflection of the diaphragm in response to pressure applied to the bottom side of the diaphragm, is formed by bonding a cap (35) to stand-offs (34) placed around the top of the diaphragm. The stand-offs are spaced to allow pressure to be applied to the top of the diaphragm.

This application is a division of application Ser. No. 676,914, filedMar. 28, 1991, U.S. Pat. No. 5,220,838.

FIELD OF THE INVENTION

The present invention relates to a pressure sensor for sensingdifferential pressure and more particularly to an overpressure-protectedsensor.

BACKGROUND OF THE INVENTION

Various single and dual direction pressure sensors are availableutilizing a silicon diaphragm which deflects in response to pressure.Deflection of the diaphragm is generally detected by sensing elementssuch as piezoresistive elements placed on the edges of the diaphragm.These sensors are generally designed so that batch fabrication ispossible. The range of pressure detection will depend on the size,thickness and span of the diaphragms.

To protect these sensors from hostile environments, the diaphragms areinsulated by an isolator arrangement which uses an incompressible fluidto transfer applied pressure from a process environment to the sensingdiaphragm. An overpressure protection device is provided to inhibit theisolator fluid from further transferring pressure to the sensor when theapplied pressure reaches a preselected limit. For example, anoverpressure protection device that isolatingly couples pressure to apair of separated volumes of substantially incompressible isolator fluidis described in U.S. Pat. No. 4,949,581 issued to Stanley E. Rud, Jr. onAug. 21, 1990. As disclosed in that patent, each volume of isolatorfluid is in fluid communication with one side of the diaphragm. Pressureapplied to the sensor is limited by two isolator diaphragms. When apreselected differential pressure limit is exceeded, the deflection ofone of the isolator diaphragms (responding to the greater pressure)bottoms against an insulator diaphragm support. Once bottomed againstthe support, no further increases in pressure are transmitted to thesensor. The pressure limits of these devices are set to protect thesensor diaphragm which has a relatively low pressure limit frompressures which will permanently deform it and thereby degrade thesensor's performance.

One method that has been suggested in order to increase the range ofsensor sensitivity and yet protect diaphragms used to measure lowerranges of differential pressures is to form a center boss on thediaphragms. When the diaphragm is exposed to excess pressure, the bossstops against a base and limits the deflection of the diaphragm beforeit is damaged. Such bosses are described in greater detail in the abovereferenced patent. A method of forming an overpressure stop bossextending from a diaphragm is similarly disclosed in U.S. Pat. No.4,790,192 issued to Thomas A. Knecht et al.

A disadvantage of this design is that the deflections and thussensitivity of the diaphragm can be affected by the overpressure boss.Diaphragms are generally less flexible in the areas of the boss andtherefore likely to be less sensitive to pressure. In order to achievethe same pressure range, the diaphragm area would have to be larger thana diaphragm area without the center boss and thus some useful siliconreal estate is wasted. Further, with extreme pressures, the unsupportedportions of the diaphragm can rupture. Thus, the extended range of thesepressure sensing diaphragms is limited.

An alternate design for a bi-directional pressure sensor, which curessome of the shortcomings of the design mentioned above, is disclosed inU.S. Pat. No. 4,905,575 issued to Knecht et al. on Mar. 6, 1990.According to the teachings of Knecht et al., a silicon diaphragm ismounted between two glass base plates which have recesses formed thereinto receive the diaphragm and provide support across the diaphragm underoverpressure conditions. The support plates serve as positive stops whenthe diaphragm is subject to overpressure and thus prevent overstressingthe diaphragm. The pressure sensor disclosed in this patent furtherincludes a diaphragm having grooves formed on opposite surfaces todefine a center deflecting portion. The grooves provide a "free edge"effect which reduces bending stress at the diaphragm edge and permit ahigher operating pressure without breakage.

Removing material to provide grooves on opposite surfaces of thediaphragm, however, requires tight control tolerances duringmanufacture. Precise alignment of the glass base supports duringassembly is also critical, especially when the sensor has an array ofsensing diaphragms with different sensing ranges. Further, glass andsilicon differ in strength and thermal coefficients. When the sensor isintended for applications over wide temperature and pressure ranges, thematerial property mismatch can create stresses and large sensing errorswhich may be difficult to overcome.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved overpressure-protected silicon chip sensor usingintegrated-circuit batch processing techniques.

Another object of the invention is to manufacture an array ofoverpressure-protected sensors having different ranges of pressuredetection using integrated-circuit batch processing techniques.

A still further object of the present invention is to provide featureswhich prevent the diaphragm from sticking to the overpressure stopsafter pressure is released.

It is a further object of the invention to provide flexiblemanufacturing and assembly tolerances to ease production ofoverpressure-protected sensors.

It is a further object of the invention to provide an improved and lesscostly method of manufacturing an array of sensors having overpressureprotection means.

The present invention results from the realization that single andbi-directional, overpressure-protected sensors can be manufactured bydepositing diaphragm material over spacers which are subsequentlyremoved to form cavities in a single surface of a semiconductorsubstrate. For overrange pressure conditions, the uniform cavity surfaceserves as a pressure stop. A second pressure stop, when required, isprovided by a cap positioned over the diaphragm and bonded to supportivestandoffs selectively deposited about the diaphragm. It was furtherrealized by the present inventors that a bi-directional,overpressure-protected sensor manufactured in this manner is relativelyeasy to manufacture because it requires wafer bonding only on one side.

In accordance with the preferred embodiment of the present invention, abi-directional, overpressure-protected sensor is constructed by firstdepositing a polysilicon diaphragm layer over an oxide spacer formed ina cavity created in the surface layer of a silicon substrate. This oxidespacer will later be etched from the backside of the substrate to definea diaphragm that is free standing and unsupported over the cavity. Aport is created by etching the backside of the substrate to remove theoxide spacer and provides pressure access to the underside of thediaphragm. This is hereinafter referred to as the "backside etch"approach. Alternatively, the oxide spacer can first be etched from thefront side and then sealed as a vacuum cavity. The backside port canthen be etched until it reaches and reopens the cavity. This approachwill hereinafter be referred to as the "punch through" approach.

The surface area of the substrate forming the cavity is used as a first,or forward, overrange pressure stop to prevent damage to the diaphragmwhen excess pressure is applied to the top side of the diaphragm. Thissurface area of the cavity is preferably coated with silicon dioxide orsilicon nitride to prevent the diaphragm from sticking to the pressurestop during overpressure conditions. This coating can be deposited oncethe backside port is etched or once the cavity is etched in thesubstrate. Non-sticking surfaces for the diaphragm and stops areimportant since in silicon sensor fabrication, surfaces are usuallyatomically smooth and can easily stick to one another permanently.

An advantage of forming a diaphragm over a cavity created in the surfacelayer of the substrate is that the cavity provides a built-in pressurestop. The diaphragm can only be deflected by a distance set by thecavity depth before reaching and being stopped by the silicon substrate.Tests conducted by the present inventors on the overrange behavior ofsealed-cavity polysilicon diaphragms have demonstrated outstandingmechanical strength and reproducibility of these devices for pressuresensing. Further, when pressure ports are on opposite sides of the chip,the sensor is easier to mount and package. Manufacturing abi-directional, overpressure-protected, differential pressure sensor inaccordance with the present invention also introduces less mechanicalparasitic effects than other prior art devices.

A second, or back pressure, overrange pressure stop is constructed byselectively depositing standoffs having a predetermined height aroundthe top side of the diaphragm. The standoffs are spaced to providesupport for a cap, which will serve as a second pressure stop, and toallow pressure to be applied to the top side of the diaphragm. Anadvantage of providing this second overrange pressure stop is that,during wafer fabrication, a large cap wafer can be easily layered ontoand bonded to the top of a wafer fabricated with multiple pressuredevices without concern for aligning the two wafers. Thereafter, thecomposite wafer can be diced into pressure sensing chips. Anotheradvantage is that the above method for constructing this bi-directionaloverrange-protected pressure sensor lends itself to a relativelyinexpensive way to manufacture multiple sensing devices on a single chiphaving a range of pressure sensitivities.

DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following and more particulardescription of the preferred embodiments of the invention, asillustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

FIG. 1 is a cross-sectional view through a silicon substrate on whichlayers of silicon dioxide and silicon nitride have been deposited.

FIG. 2 is a cross-sectional view as in FIG. 1 after etching a portion ofthe silicon dioxide and silicon nitride to form an open area definingthe perimeters of a cavity to be formed in the silicon substrate.

FIG. 3 is a cross-sectional view through the substrate after growing afirst layer of silicon dioxide on the substrate.

FIG. 4 is a cross-sectional view through the substrate showing a cavityin the substrate after the oxide is removed.

FIG. 5 is a cross-sectional view through the substrate showing thecavity of FIG. 4 filled with oxide, the top surface is level with thesurface of the substrate.

FIG. 6 is a cross-sectional view through the substrate after furtheretching the nitride/oxide layer to open a window for exposing thesilicon substrate at places where the diaphragm is to be anchored to thesubstrate.

FIGS. 7-11 are cross-sectional views through a substrate illustratingprocess steps for forming a bi-directional, overpressure-protectedsensor, in accordance with the present invention, using a punch throughmethod for reopening a sealed-cavity diaphragm sensor.

FIGS. 12-15 are cross-sectional views through a substrate illustratingalternative process steps for forming a bi-directional,overpressure-protected sensor, in accordance with the present invention,using a backside etch approach for removing a sacrificial spacer to forma diaphragm.

FIG. 16 is a perspective view of the top of a sensor module formed on awafer and having one differential pressure cell in accordance with thepresent invention.

FIG. 17 is a perspective view of the top of an alternative sensor moduleformed on a wafer and having multiple differential pressure cells forproviding a range of pressure sensing sensitivities.

FIG. 18 is a perspective view of a cap wafer bonded to a sensor wafer toform a wafer composite of bi-directional, overpressure-protected sensormodules.

FIG. 19 is a perspective view of a bi-directional,overpressure-protected sensor module showing tabs to be broken off toexpose metalized pads for external connection.

FIG. 20 is a perspective view of the bi-directional,overpressure-protected sensor module of FIG. 19 once the tabs have beenremoved.

FIG. 21 is a perspective view of an embodiment of a cap wafer havingholes formed through the wafer before it is bonded to a sensor wafer toexpose metalized pads on the sensor wafer.

FIG. 22 is a perspective view of a composite wafer formed by bonding thecap wafer shown in FIG. 21 and a sensor wafer.

FIG. 23 is a perspective view of a resulting bi-directional,overpressure-protected sensor module when the composite of FIG. 22 isdiced into individual modules.

DETAILED DESCRIPTION OF THE INVENTION

An overpressure-protected, differential pressure sensor 37 is formed bydepositing diaphragm material 24 over a cavity 23 formed and filled withsacrificial material 22 into a front surface of a substrate. Thesacrificial material 22 is then removed to create a free diaphragm. Thefloor of the cavity 23 defines a first, or forward, pressure stop tolimit the deflection of the diaphragm in response to pressure applied tothe top of the diaphragm. A port 33 is created to allow pressure to beapplied to the bottom side of the diaphragm 24. An optional second,orback pressure stop, which limits the deflection of the diaphragm inresponse to pressure applied to the bottom side of the diaphragm, isformed by bonding a cap 35 to standoffs 34 placed around the top of thediaphragm. The standoffs are spaced to allow pressure to be applied tothe top of the diaphragm.

Referring now to the drawings in detail, the processing steps which maybe utilized to produce single and dual direction, overpressure-protectedsensors in accordance with the present invention will now be described.According to the preferred embodiment, a cavity area is defined as anoxide spacer which is grown into the substrate surface. A diaphragmlayer, later deposited, will then be substantially coplanar with itssupporting substrate portions. With reference to FIG. 1, the startingmaterial comprises a crystalline silicon substrate 10. A 400 angstromthick layer of silicon dioxide 12 is thermally grown on the top andbottom sides of the wafer. A second thin layer of low-stresslow-pressure chemical vapor deposited (LPCVD) silicon nitride 14, about900 angstroms thick, is then deposited. For example, the nitride may bedeposited from a gas phase comprising a 5 to 1 or 6 to 1 ratio mixtureof dichlorosilane gas and ammonia. As exemplary conditions, the ammoniagas may be provided at a flow rate of 9 milliliters per minute and thedichlorosilane at a flow rate of 54 milliliters per minute, at apressure of approximately 150 milliTorr and a deposition temperature of800° C.

A layer of photoresist is now applied to the top of the wafer. Usingphotolithography, the photoresist is then patterned to expose thenitride 14 in the cavity area. The exposed nitride layer is etched in aCF₄ /O₂ plasma, followed by a HF etch of the oxide layer 12 to form anopen area 16 defining the perimeters of the cavity, as shown in FIG. 2.The silicon dioxide layer forms an etch stop for the nitride etchant.

The substrate is then oxidized at, e.g., 1050° C. under wet oxygen toprovide a 7500 angstrom layer 18 of silicon dioxide, as shown in FIG. 3.The entire substrate is then immersed in a HF solution to remove theoxide 18, leaving an indentation 20 in the substrate as shown in FIG. 4.In one embodiment, the surface area of the indentation left in thesubstrate is doped with a layer of boron 21, shown in phantom, usinghigh concentration boron diffusion, to act as an etchant stop for a"punch through" approach that will be described in detail below. Thewafer is then reoxidized under the same conditions as described above togrow a silicon dioxide spacer 22 in the indentation formed in thesubstrate as shown in FIG. 5. The result is an oxide filled areaapproximately 7500 angstroms thick, which has a top surfacesubstantially level with the surface of the substrate. The nitride layer14 proves to be an effective oxidation barrier, oxidizing at less that 1angstrom a minute at a temperature of 1050° C. Using a patternedphotoresist as a masking layer, the top nitride/oxide layers 14 and 12are then etched to open a window 15, as shown in FIG. 6, which exposesthe silicon substrate at places where the diaphragm is to be anchored tothe substrate.

The oxide spacer can be etched from the front side of the substrate andthen sealed as a vacuum cavity. The method of first creating an oxidespacer underneath a polysilicon layer for the purpose of creating asealed-cavity sensor was first disclosed in U.S. Pat. No. 4,744,863issued to Henry Guckel and David W. Burns on May 17, 1988. This patentis hereinafter incorporated by reference. The backside can thereafter beetched to reopen the cavity using the punch through approach. In thealternative, the oxide spacer is removed when a port is created byetching the backside of the substrate which will hereinafter be referredto as the "backside etch" approach.

Referring now to FIGS. 7-15, the punch through and backside etchapproaches will now be described. For the punch through approach(depicted in FIGS. 7-11) the anchor regions and the remainingoxide/nitride regions are patterned in such a way that the oxide/nitrideregions are interlaced to form a network of channels 19 leading to thediaphragm region 24a from the surrounding anchor areas 25. This channelnetwork will allow the etchant to enter the channels and dissolve theoxide spacer 22. A more detailed description of channel placement can befound in Guckel's patent referenced above. Using the backside etchapproach, which is depicted in FIGS. 12-15 there is no need for achannel network to remove the oxide spacer.

A layer 24 of LPCVD polysilicon (e.g. 2 micrometers thick) is thendeposited on the substrate 10, with the portion of the polysilicon layeroverlying the oxide spacer 22 (FIGS. 7 and 12). For example, a 2 micronthick layer of polysilicon may be deposited from silane gas at 580° C.at 300 milliTorr for 340 minutes and then annealed at 1150° C. for 3hours to reduce the residual strain field.

The next step in the punch through approach involves the etching of theoxide spacer to form a cavity 23. Window cuts in the polysilicon layer(not shown) are first made in an area laterally displaced from thediaphragm by using plasma or reactive ion etching until it reaches theunderlying nitride/oxide channel regions. The wafer is then immersed inHF etchant which would enter through the polysilicon cuts to startetching the channels and work its way to clear the oxide spacer. Typicalduration of the etch is 20 hours, depending on the span of thediaphragm. Afterward, the wafer is rinsed thoroughly in water.

For both the punch through and the backside etch approaches, a layer 27(FIGS. 8 and 12) of silicon dioxide, 500 angstroms thick, and a layer 29of LPCVD silicon nitride, 900 angstroms thick, are then grown. Thedesired low stress nitride layer can be achieved by using larger ratiosof dichlorosilane to ammonia. This serves as dielectric isolation uponwhich the resistors will be placed. For the punch through approach, thisstep also seals the open cavity 23 because the open channels, which areapproximately 1400 angstroms high, will be completely plugged by thegrowth of an oxide/nitride layer 30 (FIG. 8). The residual reactivegases trapped inside the cavity 23 will continue to react untilessentially a vacuum is left in the cavity. Furthermore, because of thiscoating of oxide/nitride 30 formed inside the cavity, the diaphragm willnot stick to the surface area of the cavity during overpressureconditions.

Immediately after the deposition of the silicon nitride layer 29, thewafers are transferred to a LPCVD polysilicon deposition system where alayer of sensing resistor material is deposited. In the preferredembodiment four serpentine resistors 39 (as shown in FIGS. 16 and 17)are mounted symmetrically about the diaphragm center and connected inseries to provide the highest pressure sensitivity and the best matchingof resistors from diaphragm to diaphragm. Formation of such resistors isshown and described in the patent issued to Guckel et al. referencedabove.

Generally, the resistors 39 are formed by first depositing a layer ofresistor material such as polysilicon 31 (e.g., about 5000 angstromsthick) on the nitride layer 29. An ion implant of a single dopant isapplied to the entire polysilicon layer. A photoresist is then appliedto areas of the polysilicon layer to expose contact areas andturn-around points of the polysilicon resistors. A further implant ofthe same dopant is applied to these exposed areas of the layer. Thisallows a heavier implant dosage (and hence a lower resistance) in thecontact areas and turn-around points of the polysilicon resistors. Theresistors can be doped either p-type or n-type using boron orphosphorous dopants. By utilizing both types of dopants on a singlediaphragm, a fully active resistor bridge on the diaphragm may beobtained. The turn-around points preferably have lower resistance sothat their contribution to strain sensitivity is small.

The photoresist is then removed, and another layer of photoresist isapplied to the polysilicon layer over those areas of the layer that areto be retained as resistor regions. An etchant, for example, CF₄ /O₂plasma, is then applied to the wafer to etch away the polysilicon in allareas except that covered by the photoresist layer. Thereafter, a layerof nitride 32 is deposited across the wafer.

The sensor wafers are now ready for opening the backside port. Notethat, during the fabrication of the differential pressure sensor, allthe deposition steps mentioned above may be deposited on both sides ofthe substrate. Here, it is important that the two previous LPCVDpolysilicon layers, namely the diaphragm and resistor layers, becompletely removed from the backside with plasma etching to preventinterference with the silicon etching, described below, for forming abackside port 33.

Referring now to FIGS. 9 and 13, a photoresist is first applied to thewafer backside, for defining a window area (not shown) in theoxide/nitride layers 12 and 14 for the backside port. Next, theoxide/nitride dielectric layer is etched using plasma etching and HFsolution until bare silicon substrate is exposed. For both the punchthrough and backside etch approaches, the substrate is placed in ananisotropic etchant, e.g. KOH, to etch the exposed substrate siliconuntil it reaches approximately one micrometer from the cavity as shownin FIG. 9 and the bottom of the silicon dioxide spacer as shown in FIG.13. Note that for the punch through approach (FIG. 9), the boron etchstop 21 limits the progression of the etch near the cavity floor. Forthe backside etch approach, the etching is almost self-stopping sincethe etch rate of silicon dioxide in KOH is approximately two orders ofmagnitude lower than the corresponding etch rate for crystalline siliconsubstrate. Furthermore, the front side of the wafer as well as unopenedareas on the backside will not be attacked because of the presence ofthe protective nitride layers 14 and 32. Because of the anisotropicnature of the etch, the opened port 33 takes the final shape of apyramid having its apex cut off at the etch stop. At this point, thewafer will be withdrawn from the etching bath and be thoroughly rinsedand cleaned.

Typical KOH etch conditions for both approaches are 50% weight of KOH inwater at 90° C. For a wafer 390 micrometers thick and a backside openingstarting as a 590 micrometers square on the wafer back surface, theetched silicon tapers to around 60 micrometers at the etch stop 21 orthe oxide spacer 22. The total etch time is approximately 3 hours.

In some applications, high backpressures may require installation of anoverrange stop on top of the diaphragm, however, it should be noted thatthese polysilicon diaphragms are stronger than most other silicon baseddiaphragms and will not crack or fail when subjected to moderatebackpressure. For example, sensors similar to those shown in FIGS. 11and 15, but without the dual overrange cap discussed below, have beentested to withstand 6,000 psi of forward pressure and 145 psi of backpressure with 2 micron diaphragms. If dual overrange protection isrequired, standoffs 34 are next formed in strategic locationssurrounding the diaphragm for supporting a cap 35 (FIGS. 10 and 14),which will serve as a second pressure stop. The standoffs are placedaround the diaphragm to allow pressure to be applied to the top side ofthe diaphragm. (Also see FIGS. 16 and 17.) A detailed description of thecap and location of the standoffs will be described in greater detailbelow.

In the preferred embodiment, the standoffs are formed by depositingLPCVD polysilicon with 100% silane at 580° C. and at 300 milliTorr.Following deposition, a photoresist is applied to protect those areas tobe retained as standoffs and the unprotected polysilicon layer is thencompletely removed by plasma or reactive ion etching. The underlyingnitride layer 32 will be an etch stop for this process. The polysiliconlayer forming the standoffs 34 must be sufficiently thick so that thetops of the standoffs 34 are uniformly the same height and the highestsurface on the wafer. This height will define the maximum travel of thedeflecting diaphragm. Since different travel distances will result indifferent degrees of over-pressure protection, the actual thickness ofthe polysilicon will vary according to the application of the sensor. Atypical value is about one micrometer. Note that the length and width ofthe standoffs are important for supporting the cap. The standoffs shouldalso be spaced from each other to allow pressure to be applied to thetop side of the diaphragm.

Metalized electrical contact pads 36 for interconnecting the heavilydoped portions of the polysilicon resistor layer and for providingexternal electrical connections can now be formed. A method for formingthese pads is described in detail in the Guckel patent referenced aboveand can be followed here. Generally, a photoresist layer (not shown) isfirst applied to and patterned on the top of the wafer leaving areas ofthe top nitride layer exposed where the contacts are to be formed. Anetchant is then applied to the wafer to etch through that portion of thenitride layer to expose the polysilicon resistor layer. The photoresistis then removed, and a high temperature metal system (e.g.,aluminum/titanium-nitride/titanium) is deposited over the exposedsurface, including portions (not shown) where the metal makes contactwith the heavily doped portions of the polysilicon layer at the contactregions.

The surface of the metal is then patterned with a photoresist layer andan etchant applied to etch away the metal layer not covered by thephotoresist. The photoresist is thereafter removed to leave the sensorstructure with the metalized conduction layers in the proper pattern.The metal/polysilicon contacts will be annealed during the wafer bondingstep. (See FIGS. 16 and 17 which show metalized patterns for both asingle differential pressure cell and an array of differential pressurecells.)

Referring now to FIGS. 10 and 14 a constructed sensor 37 is ready forbonding a wafer cap 35 to the standoffs 34. The cap will serve as asecond overrange pressure stop. Note that this second overrange pressurestop is not always needed for measurement of differential pressures. Apreferred bonding approach uses a boron-doped oxide (B₂ O₃) as a bondingagent. A one micrometer thick layer 38 of silicon dioxide is first grownon the silicon cap 35 which is approximately 390 micrometers thick.Next, the oxidized cap wafer is doped with boron in a diffusion furnaceat 1075° C. in a wet ambient for three hours. Immediately afterdeposition, the wafer cap 35 and the sensor 37 are brought together. Thecomposite wafer is then heated to 550° C. in an inert ambient whichcauses the boron-doped oxide to reflow. A strong bond between the capwafer and the polysilicon standoffs on the sensor results. No alignmentbetween the sensor and cap is required.

A wide variety of bonding agents other than boron-doped oxide is alsoacceptable. For example, low temperature spin-on-glass (and, inparticular, photoresist patternable spin-on-glass) flows at temperaturesbetween 500° and 600° C. A layer of spin-on-glass can be applied ontothe nitride layer 32 and patterned to form glass bumps 40 (shown inphantom in FIG. 10) away from the standoffs and active diaphragm areas.These glass bumps 40 are initially slightly higher than the standoffs.When the cap 35 and sensor 37 are pressed together at 550° C., the glassreflows and spreads out, forming a strong bond. Another preferredbonding method is the use of metal to silicon bonds. For example, alayer of aluminum film (on the order of a few tenths of micrometers) canbe vacuum deposited onto the surface of the sensor wafer and photopatterned to leave aluminum on top of the standoffs. When the cap andsensor wafers are pressed together and heated, at a temperature around600° to 650° C. in vacuum, the aluminum will alloy with the cap wafer toform a tight bond. Other bonding agents are possible. For example,anodic glass-to-silicon bonds, bonding using polymeric adhesives, aswell as many others may be used.

Upon completion of wafer bonding, the next step of processing depends onthe approach used in etching the diaphragm cavity. For the punch throughapproach, the backside of the bonded sensor wafer is etched withreactive ion etch to punch through the boron doped film 21 of substratesilicon and the oxide/nitride layer 30 blocking the opening of thebackside port to the vacuum-sealed cavity 23 as shown in FIG. 11. Sincereactive ion etching does not etch significantly in the lateraldirection, no damage to the diaphragm cavity outside the immediate areaof the opening port 33 will result.

For the backside etch approach, the composite wafer is immersed in HFsolution to remove the oxide spacer 22 to open cavity 23 and thus freethe diaphragm. The duration of the etch depends on the size of thediaphragm and the way it branches out from the backside port 33. Atypical etch time is approximately 48 hours. To protect the top of thesensor 37 from being attacked by the HF etchant, a protective material,e.g. wax, is applied to seal around the edge of the composite wafer.After the etch, the composite is immersed in a warm concentrated nitricacid to grow a native oxide coating 42 inside the port 33 and the cavity23 walls (FIG. 15). This coating prevents the diaphragm from sticking tothe walls of the cavity during overpressure conditions. Finally, the waxis stripped with solvents.

FIGS. 11 and 15 show completed dual overrange protected sensors 41. Ifonly one-sided overrange protection is required, elements 34, 35 and 38are deleted.

The above described integrated circuit batch processing techniques canbe applied to wafer fabrication of single or bi-directional,overpressure-protected sensors having a single differential pressurecell or an array of differential pressure cells for providing a widerange of pressure detection. By fabricating an array ofover-pressure-protected sensors on a single wafer, manufacturing costscan be significantly reduced because it only requires wafer bonding onone side. For example, FIGS. 16 and 17 show two possible sensor modulesthat can be fabricated in an array on a single wafer.

Referring now to FIG. 16, an array of single differential pressure cellscan be formed on a sensor wafer 43 in accordance with the presentinvention as shown. Broken lines 44 and 46 are shown to indicate theborders of a single module 48 and where the wafer will be cut once thefabrication is completed. Note that when bi-directional overrangeprotection is required, standoffs 34 are placed around a diaphragm 49,shown by broken lines, to support a wafer cap (not shown). The metalizedpads 36 which are connected to the ends of a serpentine resistor 39 willbe exposed for external electrical connection.

A possible layout of a module 50 fabricated on a sensor wafer 43 havingan array of differential pressure cells is shown in FIG. 17. Each cavityof an array of differential pressure cells can have its own back openingor they can share a common opening. The module shown employs a commonback opening 52 for the array of differential pressure cells and will bediscussed in greater detail below. Note that this opening is only shownfor clarity of the present discussion and would otherwise not beapparent to someone viewing the sensor wafer 43 from the top plan view.The outline of the module 50 is shown in broken lines 54, 55, 56 and 57,and generally indicates where wafer 43 will be cut for separating theindividual sensor module. Four differential pressure cells 58, 60, 62and 64 are fabricated between two rows of electrical pads 36. Standoffs34 have been strategically placed on the sensor wafer 43 to economizethe number of supports needed to support a wafer cap that will cover thedifferential pressure cells of each module and provide dual overrangeprotection. Note that, although the number and location of the standoffscan vary, they are placed between the two columns of metalized pads 36which will be exposed for electrical connection.

In this particular layout, the span of the four diaphragms (shown inphantom) for each differential pressure cell vary in size fromapproximately 120 to 320 micrometers square corresponding to afull-scale pressure sensor having a range from 50 to 1 psid,respectively. Serpentine resistors 39 are mounted symmetrically aboutthe center of each of the diaphragms and are serially connected tometalized pads 36 as shown. A fifth serpentine resistor 66 has beenincluded to provide a reference resistor and temperature element. Othermodule configurations are also possible. For example, the module cancontain an array of differently sized vacuum-sealed cavity devices tomeasure the absolute value of the static pressure and/or pairs ofdifferently sized differential pressure cells for providing ameasurement validation scheme. Unported diaphragms can also be providedfor calibration and measurement validation. The size of the module shownin FIG. 17 is approximately 1.0 by 1.8 millimeters.

The common back opening port 52 used to interconnect the cavity of eachof the differential pressure cells, includes a small cavity or Plenum 68(shown in phantom) formed in the silicon substrate having channels 70(also shown in phantom) radially extending from the cavity 68 to each ofthe larger cavities of the differential pressure cells. The small cavity68 is formed in the substrate at the same time as the cavities for thedifferential pressure cells are created using the same methodsdescribed. The preferred size of this cavity is about 120 micrometerssquare. Channels, extending radially to each of the other largercavities, are formed by properly masking the nitride/oxide layers duringthe process of forming the cavities. The preferred size of thesechannels is 0.75 micrometers in depth and approximately 35 micrometersin width. These channels are filled with the same sacrificial oxidewhich is removed at the same time the oxide in the cavities is removed.

Sensor wafer 43 is essentially complete and provides an array ofoverrange protected sensors. If dual overrange protection is required,then cap 35 is assembled to standoffs 34. Once the cap wafer 35 has beenbonded to the standoffs 34 using the methods described above, thecomposite wafer is then ready for dicing into individual chips (FIG.18). To accomplish this, the wafer cap is scribed in the Y direction asindicated by broken lines 72 to cut approximately 80% through the wafercap. The placement of this cut is such that tabs 74 and 76 (FIG. 19) arecreated to overhang the metalized pads 36 of the module when thecomposite wafer 88 is diced in the x and y direction as indicated by thesolid lines 78. The tabs 74 and 76 are then broken off by mechanicallybending and breaking them free to expose the electrical pads (FIG. 20)to complete the fabrication process. The module is now ready fortesting.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention, as defined by the appended claims. For example, referring toFIG. 21, holes 80 can be cut in the cap wafer 35 before it is bonded tothe standoffs 34 formed on the sensor wafer 43. When the cap wafer isbonded to the standoffs, the holes are aligned to the sensor wafer sothat the metalized pads 36 of the modules are exposed as shown in FIG.22. The composite wafer can thereafter be cut in the x and y-directionas indicated by the solid lines 82 to dice the wafer into individualmodules. A resulting module is shown in FIG. 23.

Further, both pressure ports can be manufactured on the front side ofthe substrate. In this case, at least two diaphragms communicate by aninterconnecting channel. One of the diaphragms will then be punchedopened by a dry etching technique and serve as a pressure port. Thisport is then isolated for applying pressure from one source to theinside cavity while exposing the front side of the other diaphragm toanother pressure source.

We claim:
 1. A differential pressure sensor comprisinga silicon substrate having opposed front and back surfaces, an array of cavities formed into said front surface of said substrate, at least one said cavity having a floor which defines a forward pressure stop, a polysilicon diaphragm having a top and a bottom and spanning and overlying at least said one cavity, wherein the deflection of said diaphragm in response to pressure applied to the top of the diaphragm is limited by the forward pressure stop, at least one port formed in said back of said substrate for applying pressure to the bottom of said diaphragm, and electrical means for detecting the relative deflection of the diaphragm due to a pressure differential across the diaphragm.
 2. A differential pressure sensor comprisinga silicon substrate having opposed front and back surfaces, a first cavity formed into said front surface of said substrate, said cavity having a floor which defines a first pressure stop, a deformable polysilicon diaphragm having a top and a bottom and spanning over the first cavity, wherein the deflection of the diaphragm in response to pressure applied to the top side of the diaphragm is limited by the first pressure stop, means for applying pressure to the bottom of the diaphragm, said means for applying pressure including a port formed by etching the back of the substrate, electrical means for detecting the relative deflection of the diaphragm due to a pressure differential across the diaphragm, and at least one additional cavity formed into said front surface of said silicon substrate and interconnected with said first cavity into an array of cavities.
 3. The differential pressure sensor of claim 2 wherein at least two cavities of said array of cavities are of different sizes and have diaphragms spanning and overlying said two differently sized cavities for sensing different pressure ranges.
 4. The differential pressure sensor of claim 2 further comprising,standoff means disposed about said diaphragm to support a second pressure stop, cap means disposed on said standoff means and over said diaphragm to define the pressure stop which serves to limit the deflection of the diaphragm in response to pressure applied to the bottom of the diaphragm.
 5. The differential pressure sensor of claim 2 further comprising means forming channels in said substrate connecting at least two of the cavities to a common port.
 6. The differential pressure sensor of claim 2 further comprising a coating in each cavity for preventing the diaphragm from sticking to the floor of the cavity during overpressure conditions.
 7. The differential pressure sensor of claim 6 wherein said coating is of nitride.
 8. A differential pressure sensor comprisinga silicon substrate having opposed front and back surfaces, an array of cavities formed into said front surface of said substrate, at least one said cavity having a floor which defines a first pressure stop, a diaphragm having a top and a bottom and spanning and overlying at least said one cavity, wherein the deflection of said diaphragm in response to pressure applied to the top of the diaphragm is limited by the first pressure stop, standoff means disposed about said diaphragm to support a second pressure stop, cap means disposed on said standoff means and over said diaphragm to define the second pressure stop to limit the deflection of the diaphragm in response to pressure applied to the bottom of the diaphragm, at least one port formed in said back of said substrate for applying pressure to the bottom of said diaphragm, and electrical means for detecting the relative deflection of the diaphragm due to a pressure differential across the diaphragm.
 9. The differential pressure sensor of claim 8 further comprising means forming chapels in said substrate connecting at least two of the cavities to a common port.
 10. A differential pressure sensor comprisinga silicon substrate having opposed front and back surfaces, a first cavity formed into said front surface of said substrate, said cavity having a floor which defines a first pressure stop, a deformable polysilicon diaphragm having a top and a bottom and spanning over the first cavity, wherein the deflection of the diaphragm in response to pressure applied to the top side of the diaphragm is limited by the first pressure stop, means for applying pressure to the bottom of the diaphragm, electrical means for detecting the relative deflection of the diaphragm due to a pressure differential across the diaphragm, standoff means disposed about the diaphragm to support a second pressure stop, cap means disposed on said standoff means and over the diaphragm to define the second pressure stop for limiting the deflection of the diaphragm in response to pressure applied to the bottom of the diaphragm, and at least one additional cavity formed into said front surface of said silicon substrate and interconnected with said first cavity into an array of cavities.
 11. The differential pressure sensor of claim 10 wherein at least two cavities of said array of cavities are of different sizes and have diaphragms spanning and overlying said two differently sized cavities for sensing different pressure ranges.
 12. The differential pressure sensor of claim 10 wherein said means for applying pressure to the bottom of the diaphragm includes a port formed by etching the back of the substrate.
 13. The differential pressure sensor of claim 10 further comprising a coating in each cavity for preventing the diaphragm from sticking to the floor of the cavity during overpressure conditions.
 14. The differential pressure sensor of claim 10 further comprising a coating on said cap means for preventing the diaphragm from sticking to said cap means during overpressure conditions.
 15. The differential pressure sensor of claim 10 wherein said diaphragm spans at least two cavities of said interconnected array of cavities and wherein said means for applying pressure to the bottom of said diaphragm includes a port through the diaphragm and in communication with one of said two cavities.
 16. A differential pressure sensor comprisinga silicon substrate having opposed front and back surfaces an array of cavities formed into said front surface of said substrate, each cavity having a floor which defines a forward pressure stop, a deformable diaphragm having a top and a bottom and spanning over at least one said cavity, wherein the deflection of the diaphragm in response to pressure applied to the top side of the diaphragm is limited by the forward pressure stop, standoff means disposed about the diaphragm to support a second pressure stop, cap means disposed on said standoff means and over the diaphragm to define the second pressure stop for limiting the deflection of the diaphragm in response to pressure applied to the bottom of the diaphragm, means for applying pressure to the bottom of the diaphragm, said means for applying pressure including a port formed by etching the back of the substrate, and electrical means for detecting the relative deflection of the diaphragm due to a pressure differential across the diaphragm.
 17. A differential pressure sensor comprisinga silicon substrate having opposed front and back surfaces, an array of cavities formed into said front surface of said substrate, at least one said cavity having a floor which defines a forward pressure stop, polysilicon diaphragm means having a top and a bottom and spanning at least said one cavity, wherein the deflection of said diaphragm means in response to pressure applied to said top of said diaphragm means is limited by the forward pressure stop, said one cavity being sealed pressure-wise with said diaphragm means and having a selected reduced pressure therein, and electrical means for detecting the relative deflection of said diaphragm due to a pressure differential across said diaphragm means. 